(A) Field of the Invention
This invention relates to a catalyst used for catalytically cracking ozone in an ozone-containing gas, more particularly to an ozone cracking catalyst having active manganese oxide uniformly carried in a large amount, which is excellent in ozone cracking activities and the durability of catalytic activities.
(b) Description of the Prior Arts
Since ozone has a strong oxidizing action, it is widely used for cleaning and sterilizing tap water, treating sewages or factory wastes, denitrating and deodorizing exhaust gases. However, it is normal to use an excess amount of ozone in order to fully conduct the oxidation treatment. Therefore, a superfluous amount of ozone generally remains unreacted.
Ozone is also generated from an electrophotographic copying machine employing a corona discharge system.
As is well known, ozone has a bad influence on a human body, and it is therefore necessary to remove such superfluous ozone by decomposition.
Manganese oxide is conventionally well known as a catalyst for catalytically cracking ozone, and various methods for producing the catalyst have been developed heretofore.
For example, active manganese oxide has been prepared by injecting oxygen or air into an aqueous solution of manganese sulfate in the presence of an equivalent amount of ammonia to oxidize the manganese (see Japanese Patent Laid Open No. 51-4094). It has also been reported to prepare active manganese dioxide by adding potassium permanganate to an acidic aqueous solution of a manganese salt and aging the solution (see Japanese Patent Laid Open No. 51-71299). However, none of these prior catalysts are satisfactory with respect to their catalytic activities and catalyst lives.
Japanese Patent Laid Open No. 58-183928 discloses .gamma.-MnO.sub.z prepared by three kinds of methods, and it is proposed to form the manganese oxide thus prepared into graanules or a honeycomb structure in combination with an inorganic binder or to form a catalyst coating layer by lining the catalyst on the surface of an apparatus. Also, Japanese Patent Laid Open No. 56-70823 discloses a method for carrying manganese dioxide on a carrier by Wash-coating Method. However, a satisfactory catalyst activity in proportion to the carried amount of manganese dioxide can not be achieved and its catalyst life is also not satisfactory.
Treatment of exhausted ozone is generally accompanied by the treatment of a large amount of gas, and it is therefore necessary to reduce the pressure loss across a catalyst bed. Accordingly, a catalyst supported on a carrier has been widely used heretofore. Japanese Patent Laid Open No. 53-87972 disclosed a method for impregnating active carbon with manganese oxide or previously blending manganese oxide with an active carbon starting material. Japanese Patent Laid Open No. 53-146 88 discloses a method for impregnating particulate alumina with manganese sulfate and thereafter calcining the same. In order to reduce the pressure loss across a catalyst layer, it has also been proposed to coat manganese dioxide on fillers such as Raschig rings and the like (see Japanese Patent Laid Open No. 55-73323) or to coat the surface of a monolith carrier base (see Japanese Patent Laid Open No. 56-166942). However, any of these conventional methods could not increase an effective catalyst amount, nor could improve a catalyst life.
In order to reduce the pressure loss by a catalyst and to improve the purification rate of exhaust gases, research and development concerning carrier structures supporting a catalyst thereon have progressed remarkably recently. Particularly, a honeycomb structure is known to be an excellent structure since it reduces the pressure loss and enlarges contact area. Heretofore, there are many honeycomb structures formed by extruding ceramics such as cordierite, mullite and the like.
However, conventionally known honeycomb structures prepared by sintering the above mentioned ceramics have disadvantages in that a treating gas generally flows in parallel with the wall of a cell and, consequently, it is hard to secure a large contact area. In order to enlarge the contact area, it is necessary to make the wall of a cell thinner and to increase the number of cells per unit section area, however, such a design leads to weaken the strength of a carrier structure.